The present invention relates to an acousto-optic tunable filter (AOTF) configuration which extends the tuning range of the filter. Additionally, the invention provides a method for extending the tuning range of an acousto-optic tunable filter.
The term acousto-optic filter refers to the fact that in certain birefringent optical materials, a light beam propagating as an E-ray can, under certain conditions, be converted into an O-ray by interaction with, and difraction from, an acoustic wave propagating in the same medium. This phenomenon has been utilized in producing narrow band optical filters, the peak transmission wavelength of which can be selected by properly choosing the frequency of the acoustic wave. The center wavelength of the passband of the acousto-optic filter is electronically tunable by changing the frequency of the acoustic wave within the crystal.
Two basic types of tunable acousto-optic filters have been constructed: collinear and non-collinear. A collinear acousto-optic filter is disclosed in U.S. Pat. No. 3,679,288 entitled "Tunable Acousto-Optic Method and Apparatus" by Stephen E. Harris. Harris was concerned primarily with the collinear filter, in which the incident and diffracted light beams inside the birefringent crystal are collinear with the acoustic beam. A diffracted light beam at the selected passband is separated from the incident light beam with a polarizing beam splitter. In the non-collinear filter, the light beams inside the birefringent crystal are non-collinear with the acoustic beam. U.S. Pat. No. 4,052,121 to Chang entitled "Noncollinear Tunable Acousto-Optic Filter", teaches that an electronically tunable optic filter with large angular aperture can be obtained by utilizing the interaction of optical and acoustic beams that propagate non-collinearly in an anisotropic medium. Incident light of one polarization is diffracted by the acoustic wave in an orthogonal polarization over an optical passband, the center of which can be tuned by changing the acoustic frequency. U.S. Pat. No. 3,679,288 and U.S. Pat. No. 4,052,121, which are identified above, are incorporated herein by reference as if the contents thereof were completely set forth herein.
Both collinear and non-collinear filters possess unique advantages and liabilities. In general, the collinear acousto-optic tunable filter will yield a higher resolution but the extraction of the filtered light output will require a polarizing beam splitter. The non-collinear acousto-optic tunable filter is often more convenient to use because there is some angular separation between the incident and the filtered light. It may be difficult to achieve high resolution with non-collinear filters of reasonable geometry, but for many applications, the achievable results are more than adequate. One such particularly useful application of the non-collinear acousto-optic tunable filter is described in U.S. Pat. No. 4,490,845 entitled "An Automated Acousto-Optic Infrared Analyzer System", which is assigned to the assignee of the present invention and incorporated herein by reference. This patent teaches an automated acousto-optic tunable filter infrared analyzer system usable in a variety of industrial and commercial control applications. The system relies upon a narrow passband tunable acousto-optic filter which is selectively tuned by predetermined RF signals to selectively transmit the narrow band-pass of interest which corresponds to a specific molecular species for identification and analysis. The system includes a microcomputer and associated memory functions to measure and compare detected signals from an infrared detector which converts the filtered infrared signal to an electrical signal. The memory provides control signals for the computer and for controlling the sequence and frequency of RF engergy applied to tune the filter. In this way, the near-to-mid range infrared can be analyzed for absorption bands corresponding to predetermined molecular species such as combustion product gases, and a feedback signal generated to control the combustion process.
The development of new efficient infrared acousto-optic materials such as thallium-arsenic-selenide (Tl.sub.3 AsSe.sub.3) as described in U.S. Pat. No. 3,792,287; thallium-phosphorus-selenide per U.S. Pat. No. 3,929,970; and thallium-arsenic-sulfide per U.S. Pat. No. 3,799,659 all of which are owned by the assignee of the present invention and are incorporated herein by reference provide the possibility of operation over the near-to-mid infrared range of from about 1.3 micrometers to about 16 micrometers. The crystal Tl.sub.3 AsSe.sub.3 (TAS) provides a relatively high figure of merit and transmits from 1.25 to 16 microns. In addition, the crystal symmetry of the TAS crystal makes it particularly suitable for use in non-collinear applications. While the TAS crystal possesses a relatively high figure of merit and extended transmission range, the optical wavelength range capability of the device is limited by the RF range capability of the acoustic transducer structure. The full optical range of a typical TAS AOTF design requires an RF bandwidth from 12.4 to 91 MHz, which cannot be readily done with a single transducer structure. The usable fractional bandwidth of the transducer will generally never exceed 100%, and will almost always be less due to practical limitations. It is often desirable to operate a single acousto-optic tunable filter with as large as possible a wavelength coverage and this may require an RF bandwidth greater than 100%. There have been several posed solutions to increasing the bandwidth capability of an acousto-optic device. For example, U.S. Pat. No. 3,759,603 discloses an acousto-optical light deflector having increased bandwidth by the use of providing three transducers along one side of the crystal, which transducers operate at consecutive frequency ranges. Such a structural configuration, however, requires the use of an optical medium of increased dimensions. Typically, as the size of the crystal increases in both length and width, problems are encountered in both the optical quality and mechanical integrity of the medium, and the device fabrication becomes more difficult.
It is, therefore, an object of the present invention to provide an acousto-optic tunable filter configuration in which the range coverage may be doubled without the corresponding increase in crystal size.